as i can remember...

if you are using "iomanip.h" then
you are using OLD version of standard library

if you use "iomanip" then it includes new standard library,
that extensively uses namespace
and all basic functions/objects are under namespace "std"
that's why you need "using namespace std;"
which make namespace "std" as default namespace
to avoid typing std.xxx all the time

integer, long short are for INTEGERS, eg. 1, -5, 29, 888
float, single, double are for real numbers, e.g -22.59301, 0.2593958
check reference manual for details

p.s.
if you still don't get it, email me if you want
sorry I can't type chinese right now
but i can type chinese at home if necessary